The 2 million figure has been used for some years now...it seems like surely it must be more than that by now...?
"There were an estimated 1,700,000 to 2,100,000 children (grades K-12) home educated during 2002-2003 in the United States. Homeschooling appears to still be the fastest-growing form of education." – Brian D. Ray, Ph.D., Facts on Homeschooling *
*Home education has constantly grown over the last two decades. The growth rate is 7% to 15% per year, according to Dr. Brian Ray, president of the National Home Education Research Institute (Worldwide Guide to Homeschooling).
Therefore, I think it'd be safe to say there should be anywhere from 2.2 Mil (1.7 Mil with an annual growth rate of 7%) to 3.6 Mil (2.1 with an annual growth rate of 15%). So to be safely in the middle, I'd estimate at just under 3 Mil. But then we also know that most studies listing homeschooling population often grossly underestimate the number of students due to a number of factors (larger families, less registration, etc).